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ARROW-5361: [R] Follow DictionaryType/DictionaryArray changes from ARROW-3144

At the moment however, all the `DictionaryMemo` use is internal, it should probably be promoted to arguments (with defaults) to the R functions.

I'll do this here or on another PR if this one is merged first so that `r/` builds again on travis.

This now needs the C++ lib up to date, e.g. on my setup I get it through `brew install apache-arrow --HEAD`, and there is no conditional compiling so that it still works with previous versions. Let me know if that's ok.

follow up from #4316

Author: Romain Francois <romain@rstudio.com>

Closes #4413 from romainfrancois/ARROW-5361/dictionary and squashes the following commits:

b0de1a8a <Romain Francois> R should pass now
2556c16f <Romain Francois> document()
fa0440f1 <Romain Francois> update R to changes from ARROW-3144 #4316